6 Answers
6

Maybe you've zoomed in/out a bit. This can happen either accidently or knowingly when you do Ctrl+Scrollwheel. Maybe it isn't completely resetted to zoom level zero. This mis-rendering behaviour is then recognizeable at some websites, also here at SO.

To fix this, just hit Ctrl+0 or do View > Zoom > Reset to reset the zoom level to default.

This is Firefox/Gecko bug 410959. It's from 2008 and there's no real progress on it, so you'll probably need to find a workaround.

you are right . . that works. but i still dont get it . .why would zooming remove some borders but not others ??
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leoraJan 14 '10 at 2:24

1

It's hard to halven the thickness of a one-pixel thick line. As said, it's just a mis-rendering behaviour.
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BalusCJan 14 '10 at 2:27

As an example, try to zoom out the SO page you're facing now. You'll see that the tabs oldest, newest and/or votes here above will virtually lose their border-bottom at certain levels.
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BalusCJan 14 '10 at 2:34

interesting . . i guess it makes sense . .but i would still expect it to be consistent across the same size element.. so i would expect every table to lose its border consistently
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leoraJan 14 '10 at 2:51

No, surely not. If you shrink from e.g. 800px height to 799px height, then there's still only 1px line which can be dropped.
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BalusCJan 14 '10 at 11:17

I found a similar problem when zoomed out in Firefox on an application I am working on.

The main cause was having the CSS border-collapse property set to collapse.

Changing it to separate instead fixed the problem. However, that meant I did have to rethink the way borders are applied to various parts of the table, because otherwise your borders will appear to double in thickness. I ended up having to use jQuery to give a special "last" class to the last td or th in each tr, and to the last tr in the table. My query was something like: